1. Field of the Invention
This invention relates to a wide angle photographic lens, and more particularly to the focusing mechanism thereof.
2. Description of the Prior Art
Generally, focusing of a photographic lens to an object is effected by the so-called entire system shifting, wherein the entire lens system is moved relative to the film surface. However, the movement of the entire lens system requires the movement of an object having a great weight and, therefore, the structure of the lens barrel unavoidably becomes complicated and bulky. Also, in this case, variations in aberrations have been great and it has been impossible to maintain a sufficient image forming performance for objects at short distances. It is therefore known to vary a certain lens interval when focusing on a short distance object is effected. For example, U.S. Pat. No. 4,029,397 discloses separating the lens system into a forward group and a rearward group and effecting focusing by the movement of the rearward group alone. In this case, astigmatism is surely maintained substantially well corrected, but distortion remains to the same degree as that experienced in the conventional entire system shifting lens, or is increased above that in the negative direction. Since distortion is increased in proportion to the cube of the angle of view, variation in distortion is great and non-negligible in a wide angle lens. Moreover, in the wide angle lens disclosed in U.S. Pat. No. 4,029,397, the rearward group includes a relatively large number of lenses and, therefore, the moving mechanism has unavoidably become bulky and complicated.